1. Field of the Invention
The present invention relates to an apparatus and a process for supplying recirculated exhaust gases to incoming air of a piston-type internal combustion engines; for example, Otto and diesel engines.
2. Background
Exhaust gas recirculation, also referred to by the abbreviation EGR, is known in association with internal combustion engines as a common method for beneficially influencing fuel combustion. EGR implies that a part of the total exhaust-gas flow out of the engine is recirculated, and this recirculated part-flow is introduced back into the inlet side of the engine where it is mixed with incoming air prior to entering the engine cylinders. In this way, it is possible to reduce the quantity of nitrous oxides (NOx) in the exhaust gases released into the environment. This technology has been used for a relatively long time in association with otto-type engines, but interest in the process in relation to diesel engines has also grown. The technology has especially been used in vehicle applications in which the environmental requirements are relatively stringent; but with generally increasing environmental demands, interest in EGR technology is also increasing within shipping and industrial applications, for example.
The share of exhaust gas in the air/exhaust-gas mixture that is supplied to the engine cylinders has to be precisely controlled, since too small an exhaust-gas component normally produces an increased NOx production and too large an exhaust-gas component can cause a heavy increase in sooting. In order to achieve low NOx and soot emissions, it is not only important that the total exhaust-gas component be optimized, but also that the exhaust-gas component be equally large in all of the cylinders. In terms of engine wear, for example at the pistons, piston rings, linings and bearings it is important that the exhaust-gas component be the same in all cylinders. In order to obtain this even distribution of the exhaust-gas component to the various cylinders, it is important that the recirculated flow of exhaust gases be suitably mixed into the incoming air.
For simplicity's sake, in the continuation of this description the notation “EGR-flow” shall be used in a number of places for the recirculated part-flow of the total exhaust-gas flow of the engine. In addition, “EGR-pulse” denotes a pulse in the part-flow and “exhaust-gas pulse” a pulse in the total exhaust-gas flow, unless otherwise evident from the context.
Each time the exhaust valves of the cylinders are opened, a pressure pulse is created in the exhaust system resulting in an increase in the EGR-flow. In a standard internal combustion engine of the four-stroke type, the exhaust valve of the cylinder is opened every other engine revolution. Therefore, in a six-cylinder engine, for example, there are three exhaust-gas pulses per engine revolution. If the exhaust-gas branches are divided to serve three cylinders each and the EGR-flow is taken from both the exhaust-gas branches, an EGR-flow with three pulses per engine revolution is consequently obtained. If the EGR-flow is taken from one of these branches, three EGR-pulses are instead obtained for every two engine revolutions from the same engine. Depending on the engine design, the EGR system can be variously configured and the number of EGR-pulses per engine revolution can thus be lower than the total number of exhaust-gas pulses per engine revolution. The important thing, from a mixing technology viewpoint, is that the EGR-flow that is to be mixed with incoming air should be a pulse flow.
If the pulse-shaped EGR-flow is supplied to incoming air without any special mixing measures, the EGR-flow will be mixed poorly into the air resulting in the air containing “clouds” or pockets of exhaust gas. The exhaust-gas component in the air/exhaust-gas mixture (gas mixture) that is supplied to a particular cylinder will then depend on how the gas mixture outside the cylinder happens to be composed at the moment when the induction valve of the cylinder is opened. Even if the share of exhaust gases in the gas mixture, viewed in total for the entire engine, is of the desired proportion, it is very likely that the share in the various cylinders will be either too low or too high.
Usually the EGR-flow is recirculated to incoming air by a small feed pipe being connected in a known manner to the induction air duct, for example to the induction pipe directly before branch-off to the cylinders. A known method for reducing the effect of the above-mentioned “cloud” formation is to produce turbulent flows at or after the connection by, for example, using a system of small guide plates, also referred to as “turbulators,” or by using various types of venturi devices. Such venturi devices utilize an underpressure in the air and can be configured, for example, such that the feed pipe is connected to a narrowed section of the air duct in which an increased airflow velocity results in a lower static pressure. In JP 200000896, an example is shown of a known technology in which turbulators are used and U.S. Pat. No. 5,611,204 discloses a number of different venturi devices. Venturi devices are known for producing a relatively good mixture of each individual exhaust-gas pulse in the incoming air. The effect of the pulsing of the EGR-flow remains, however, since the “clouds” of exhaust gas in the air/exhaust-gas mixture are well separated in the flow direction of the air current. This means that the share or proportion of exhaust gas in the gas mixture that is sucked into the various cylinders can still vary considerably and cause the problems discussed above. Apart from this drawback, many venturi devices are far too bulky to be suitable for utilization, for example, in cramped engine compartments of heavy vehicles; and moreover, such devices are relatively expensive to produce.